Accumulator having operating fluid volume independent of external hydrostatic pressure

ABSTRACT

A method and device for maintaining pressure in an accumulator for subsea wellbore operations is disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

Accumulators are devices that provide a reserve of hydraulic fluid under pressure. Accumulators are used in, for example, hydraulically-operated systems where hydraulic fluid under pressure operates a piece of equipment or a device. The hydraulic fluid may be pressurized by a pump that maintains the high pressure required.

If the piece of equipment or the device is located a considerable distance from the pump, for example, a significant pressure drop can occur in the hydraulic conduit or pipe which is conveying the fluid from the pump to operate the device. Therefore, the flow may be such that the pressure level at the device is below the pressure required to operate the device. Consequently, operation may be delayed until such a time as the pressure can build up with the fluid being pumped through the hydraulic line. This result occurs, for example, with devices located in a body of water at great depth, such as with a subsea test tree (“SSTT”) and blowout preventer (“BOP”) equipment, which is used to shut off a wellbore to secure an oil or gas well from accidental discharges to the environment. Thus, accumulators may be used to provide a reserve source of pressurized hydraulic fluid for such types of equipment.

In addition, if the pump is not operating, or if no pump is used, accumulators can be used to provide the source of pressurized hydraulic fluid to enable the operation of the piece of equipment or device.

Accumulators conventionally include a compressible fluid, e.g., gas such as nitrogen, helium, air, etc., on one side of a separating mechanism in a pressure resistant container, and a substantially incompressible fluid (e.g., hydraulic oil) on the other side of the separating mechanism. When the hydraulic fluid is released from the accumulator and the system pressure drops below the pressure on the gas side of the separating mechanism, the separating mechanism will move in the direction of the hydraulic fluid side of the separating mechanism, displacing the stored hydraulic fluid into the piece of equipment or the device as required.

When a conventional accumulator is exposed to external hydrostatic pressure, such as encountered in subsea operations, the available volume of hydraulic fluid that can be discharged from the accumulator is decreased because the hydrostatic pressure must first be overcome in order to displace the hydraulic fluid from the accumulator. Once a conventional accumulator begins to displace fluid under such conditions, the pressure of the incompressible fluid decreases and eventually cannot overcome the hydrostatic pressure, thus causing the remaining fluid in the conventional accumulator to become essentially unusable. One technique known in the art for using accumulators exposed to hydrostatic pressure is to compensate for the expected hydrostatic pressure by increasing the gas charge pressure on the gas side of the separator mechanism to compensate for the hydrostatic pressure. In conventional accumulators, the amount of gas pressure (called “precharge”) must usually be selected for the operating depth in a body of water in order to optimize the available hydraulic fluid volume. In a deep subsea well, for example, the required gas precharge pressure may be higher than the hydraulic fluid pressure, rendering the accumulator useless when testing the hydraulic circuit from the surface. A conventional accumulator has the further shortcoming that it cannot be used at different depths; it must be used at the depth for which it is configured or the accumulator may still have a substantial amount of unusable hydraulic fluid.

Pressure-balanced accumulators have been proposed to overcome the above-described shortcomings of a conventional accumulator. One type of pressure-balanced accumulator is disclosed for example, in U.S. Pat. No. 6,202,753 to Baugh. Other examples of pressure balanced accumulators are disclosed, for example, in U.S. Pat. No. 7,628,207 issued to Leonardi et al. and assigned to the assignee of the present invention.

There continues to be a need for improved pressure balanced accumulators.

SUMMARY

One aspect of the invention is an accumulator for subsea wellbore operations including a generally cylindrical housing having a first and second longitudinal end each having a fluid port. The housing is divided into two sections by a bulkhead. A first piston is disposed on one side of the bulkhead and a second piston is disposed on the other side of the bulkhead. A connecting rod disposed between piston and defines an atmospheric pressure or vacuum chamber in a longitudinal end in contact with the second piston. The first piston defines an hydraulic fluid chamber and a gas charge pressure chamber between it and the bulkhead. The second piston defines a hydrostatic pressure chamber between it and the second longitudinal end of the housing. The first and second pistons having substantially equal cross sectional areas on both sides thereof such that a pressure of fluid in the hydraulic fluid pressure chamber is substantially equal to a pressure of gas in the gas charge pressure chamber and a hydrostatic pressure applied to the hydrostatic pressure chamber.

Other aspects and advantages of the invention will be apparent from the description and claims which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an example subsea wellbore with a test tree attached to the top thereof, and example accumulators according to the invention disposed in or about a riser pipe that extends to the water surface.

FIG. 2 shows a cross section of an example pressure balanced accumulator according to the invention.

DETAILED DESCRIPTION

FIG. 1 shows an example subsea wellbore 18 drilled through formations below the bottom 20 of a body of water 20. The wellbore 18 may have installed at its upper end a subsea test tree (“SSTT”) 14, shown only schematically for clarity of the illustration. The SSTT 14 may include various valves and controls (not shown separately) for controlling flow of fluids from the wellbore 18 and other functions. Hydraulic lines 16 connect to one or more accumulators 10 which may be disposed inside a riser 12 coupled above the SSTT 14. The riser 12 may extend to the surface wherein test control equipment (not shown) may be located, for example, on a floating drilling or production platform (not shown). The one or more accumulators 10 may be disposed in an annular space between the riser 12 and a production tubing 13 disposed inside the riser 12. As will be appreciated by those skilled in the art, the one or more accumulators 10 may provide hydraulic fluid under pressure to operate the various valves and controls in the SSTT 14.

An example accumulator according to the invention is shown schematically in FIG. 2. The accumulator 10 may include a generally cylindrically shaped housing 30. The housing 30 may be made from stainless steel, titanium of other high strength material that can resist both internal pressure and the hydrostatic pressure of a body of water at the depth at which the accumulator 10 is disposed during use. The housing 30 may be separated into two sections by a suitably located bulkhead 32. The bulkhead 32 may have an opening 33 generally in the center thereof to enable passage of a connecting rod 34 through the opening 33.

One end of the connecting rod 34 is in contact with a first piston 40. In FIG. 2, the left hand side of the first piston 40 defines a chamber 42 (“hydraulic fluid chamber”) that is filled with hydraulic fluid such as silicone oil. An hydraulic port 48 may be located on a first longitudinal end of the housing 30 and may be configured to enable the hydraulic fluid, when discharged under pressure, to enter the lines (16 in FIG. 1) to operate certain components of the SSTT (14 in FIG. 1). The other end of the connecting rod 34 includes a chamber 36 (“atmospheric chamber”) that may be initially at surface atmospheric pressure or may have vacuum therein. Such atmospheric chamber 36 may be in pressure communication through ports 36A to a larger atmospheric chamber 37 defined between the interior wall of the housing 30 and the connecting rod 34 before any external pressure is applied to the second piston 38 through a hydrostatic pressure port 46. The hydrostatic pressure port 46 may be disposed at the second longitudinal end of the housing 30 and is open to the fluid disposed in the riser (12 in FIG. 1). Such riser fluid in configurations such as shown in FIG. 1 will generally have a pressure related to the depth of the accumulator 10 and the density of the fluid in the riser (12 in FIG. 1). A second piston 38 may separate the atmospheric chamber 36 from a hydrostatic pressure chamber 44 defined between the second piston 38 and the second longitudinal end of the housing 30 (on the right hand side of the housing 30 as shown in FIG. 2). The hydrostatic pressure chamber 44 is open to external pressure of the riser fluid or the water (22 in FIG. 1) through a hydrostatic port 48 disposed generally on the second longitudinal end of the housing 30 and will be pressurized to the fluid pressure existing at any depth in the water (22 in FIG. 1) or the riser (12 in FIG. 1) at which the accumulator 10 is disposed. A gas charge pressure chamber 50 is defined in the annular space between the connecting rod 34 and the housing 30 longitudinally disposed between the first piston 40 and the bulkhead 32. The gas charge pressure chamber 50 may be charged at the surface to a selected pressure (e.g., 3,000 pounds per square inch) through a port 30A in the bulkhead 32 (with a passage therethrough into the gas charge pressure chamber 50).

In the present example, annular supports 52 may be disposed at selected locations within the gas charge chamber 50 to support the housing 30 and to facilitate longitudinal movement of the connecting rod 34. The annular supports 52 may include passages 54 so that the gas in the pressure charge chamber 50 may move freely therethrough when the accumulator 10 is operated

The first 40 and second 38 pistons preferably each have the same cross-sectional area on the faces thereof exposed, respectively to the hydraulic fluid chamber 42 and the hydrostatic chamber 44. The cross-sectional area of the connecting rod 34 occupies some of the cross sectional area of the interior of the housing 30. Thus, the pressure exerted on the hydraulic fluid in the hydraulic fluid chamber 42 is at a lower pressure than the gas charge pressure, such lower pressure being the product of the gas charge pressure and the ratio of cross sectional area the annular cross sectional area of the gas charge chamber 50 and hydraulic fluid chamber side of the first piston 40. The diameter of the connecting rod 34 may be selected to provide a selected hydraulic fluid pressure given a selected gas charge pressure. For example, the ratio of cross sectional areas on the gas charge pressure chamber and the hydraulic fluid chamber side of the first piston 40 thereof may be ⅗. Thus, a gas charge pressure of 5000 pounds per square inch will provide an hydraulic fluid pressure of 3000 pounds per square inch when there is no hydrostatic pressure applied to the second piston 38.

The second piston 38 has the same cross sectional area as the first piston 40 on the face thereof exposed to the hydrostatic chamber 44, and on its other side is exposed to the atmospheric chamber 36, 36A, 37 defined by the connecting rod 34, which has the same annular cross sectional area as the gas charge pressure chamber 50 as a result of some of the cross section being occupied by the connecting rod 34 (which may have a substantially constant diameter).

As the accumulator 10 is moved deeper into the water (22 in FIG. 1) or the riser (12 in FIG. 1) is moved deeper into the water, the hydrostatic pressure entering the hydrostatic port 46 increases correspondingly. Such hydrostatic pressure is exerted against the second piston 38, which causes it to move in the direction of the gas charge pressure chamber 50. Because the ratio of cross sectional areas of the first piston 40 with respect to the pressure charge chamber 50 and the hydraulic fluid chamber 42, and the cross sectional area of the second piston 38 with respect to the atmospheric chamber 36, 36A, 37 and the hydrostatic pressure chamber 44 are the same, the amount of pressure in the hydraulic fluid chamber 42 will substantially always be equal to the gas charge pressure in the gas charge chamber 50 plus the hydrostatic pressure exerted against the second piston 38. Thus, the hydraulic fluid in the hydraulic fluid chamber 42 is maintained at a pressure equal to the gas charge pressure plus the hydrostatic pressure, i.e., at a constant pressure above the hydrostatic pressure. Such constant differential pressure may provide that a substantially constant volume of hydraulic fluid may be available to be discharged from the hydraulic fluid chamber 42 irrespective of the ambient hydrostatic pressure.

A possible advantage of having the various chambers in the accumulator 10 arranged as shown in and explained with reference to FIG. 2 is that the operating ports (i.e., the hydrostatic port 46 and the hydraulic fluid port 48 are at the respective longitudinal ends of the accumulator. Thus, it is not necessary to have hydraulic or other lines exiting the side of the housing 30. Such configuration may enable the accumulator 10 to occupy less effective annular space between the riser (12 in FIG. 1) and the production tubing (13 in FIG. 1). It may thus be possible to fit a selected number of the present example accumulators within a particular diameter riser, or to use a smaller diameter riser for a given number of accumulators.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims 

1. An accumulator for subsea wellbore operations having hydraulic charge pressure maintained at a substantially constant pressure above hydrostatic pressure at any depth in a body of water, the accumulator comprising: a generally cylindrical housing having a first longitudinal end and a second longitudinal end, each longitudinal and having a port therein, the housing divided into two sections by a bulkhead; a first piston disposed in the housing on one side of the bulkhead; a second piston disposed in the housing on the other side of the bulkhead; a connecting rod disposed between the first and second pistons, the connecting rod having an atmospheric pressure or vacuum chamber defined in a longitudinal end thereof in contact with the second piston; and wherein the first piston defines an hydraulic fluid chamber between the first piston and the first longitudinal end of the housing and a gas charge pressure chamber disposed between the bulkhead and the first piston, and wherein the second piston defines a hydrostatic pressure chamber between the second piston and the second longitudinal end of the housing, the first and second pistons having substantially equal cross sectional areas on both sides thereof such that a pressure of fluid in the hydraulic fluid pressure chamber is substantially equal to a pressure of gas in the gas charge pressure chamber and a hydrostatic pressure applied to the hydrostatic pressure chamber.
 2. The accumulator of claim 1 further comprising annular supports disposed in the gas charge pressure chamber, the annular supports having openings generally in a center thereof to enable passage of the connecting rod and openings to enable flow of gas in the gas charge pressure chamber.
 3. The accumulator of claim 1 wherein the accumulator is disposed in a riser coupled to a subsea test tree, and the port in the first longitudinal end of the housing is coupled to the subsea test tree to provide hydraulic pressure to operate components thereof.
 4. The accumulator of claim 3 wherein the port in the second longitudinal end of the housing is exposed to fluid in the riser.
 5. The accumulator of claim 1 wherein a ratio of cross sectional area of the first piston exposed to the hydraulic fluid chamber and exposed to the gas charge pressure chamber is selected such that a selected gas charge pressure results in a selected hydraulic fluid pressure.
 6. The accumulator of claim 1 wherein an initial pressure of the gas charge is about 5,000 pounds per square inch.
 7. The accumulator of claim 6 wherein an initial pressure of the hydraulic fluid is about 3,000 pounds per square inch plus the hydrostatic pressure.
 8. A method for maintaining pressure in an accumulator for subsea wellbore operations having hydraulic fluid pressure maintained at a substantially constant pressure above hydrostatic pressure at any depth in a body of water, the method comprising: communicating pressure of a pressurized gas in a container to a body of hydraulic fluid and to a chamber having at least one of atmospheric pressure and vacuum therein; communicating hydrostatic pressure to the body of hydraulic fluid such that a pressure of the hydraulic fluid is substantially always equal to a sum of the pressure in the container and the hydrostatic pressure.
 9. The method of claim 8 further comprising coupling the body of hydraulic fluid to a control in a subsea test tree.
 10. The method of claim 8 further comprising discharging the hydraulic fluid to operate the control.
 11. The method of claim 8 wherein the communicating hydrostatic pressure is performed by a first and a second piston each disposed at an opposed longitudinal end of a connecting rod, the connecting rod passing through the container, wherein the hydrostatic pressure exerted on the first piston is communicated through the connecting rod to the second piston in contact with the hydraulic fluid.
 12. The method of claim 8 wherein the first piston separates compartments in an accumulator housing having the hydrostatic pressure in a first compartment and the at least one of atmospheric pressure and vacuum in a second compartment.
 13. The method of claim 8 wherein an initial pressure of the pressurized gas is about 5,000 pounds per square inch.
 14. The method of claim 8 wherein an initial pressure of the hydraulic fluid is about 3,000 pounds per square inch plus the hydrostatic pressure.
 15. An accumulator for subsea wellbore operations having hydraulic charge pressure maintained at a substantially constant pressure above hydrostatic pressure at any depth in a body of water, the accumulator comprising: a generally cylindrical housing having a first longitudinal end and a second longitudinal end, each longitudinal and having a port therein, the housing divided into two sections by a bulkhead; a first piston disposed in the housing on one side of the bulkhead; a second piston disposed in the housing on the other side of the bulkhead; a connecting rod disposed between the first and second pistons, the connecting rod having an atmospheric pressure or vacuum chamber defined in a longitudinal end thereof in contact with the second piston; wherein the first piston defines an hydraulic fluid chamber between the first piston and the first longitudinal end of the housing and a gas charge pressure chamber disposed between the bulkhead and the first piston, and wherein the second piston defines a hydrostatic pressure chamber between the second piston and the second longitudinal end of the housing, the first and second pistons having substantially equal cross sectional areas on both sides thereof such that a pressure of fluid in the hydraulic fluid pressure chamber is substantially equal to a pressure of gas in the gas charge pressure chamber and a hydrostatic pressure applied to the hydrostatic pressure chamber; and wherein the accumulator is disposed in a riser coupled to a subsea test tree, and the port in the first longitudinal end of the housing is coupled to the subsea test tree to provide hydraulic pressure to operate components thereof.
 16. The accumulator of claim 15 further comprising annular supports disposed in the gas charge pressure chamber, the annular supports having openings generally in a center thereof to enable passage of the connecting rod and openings to enable flow of gas in the gas charge pressure chamber.
 17. The accumulator of claim 15 wherein the port in the second longitudinal end of the housing is exposed to fluid in the riser.
 18. The accumulator of claim 15 wherein a ratio of cross sectional area of the first piston exposed to the hydraulic fluid chamber and exposed to the gas charge pressure chamber is selected such that a selected gas charge pressure results in a selected hydraulic fluid pressure.
 19. The accumulator of claim 15 wherein an initial pressure of the gas charge is about 5,000 pounds per square inch.
 20. The accumulator of claim 15 wherein an initial pressure of the hydraulic fluid is about 3,000 pounds per square inch plus the hydrostatic pressure. 